Thermal dye transfer methods include thermal dye sublimation transfer also called thermal dye diffusion transfer. This is a recording method in which a dye-donor element provided with a dye layer containing sublimating dyes having heat transferability is brought into contact with a receiver sheet and selectively, in accordance with a pattern information signal, heated with a thermal printing head provided with a plurality of juxtaposed heat-generating resistors, whereby dye is transferred from the selectively heated regions of the dye-donor element to the receiver sheet and forms a pattern thereon, the shape and density of which are in accordance with the pattern and intensity of heat applied to the dye-donor element.
A dye-donor element for use according to thermal dye sublimation transfer usually comprises a very thin support e.g. a polyester support, one side of which is covered with a dye layer comprising the printing dyes. Usually, an adhesive or subbing layer is provided between the support and the dye layer. Normally, the opposite side of the support is covered with a slipping layer that provides a lubricated surface against which the thermal printing head can pass without suffering abrasion. An adhesive layer may be provided between the support and the slipping layer.
The dye layer can be a monochromic dye layer or it may comprise sequential repeating areas of differently coloured dyes e.g. dyes having a cyan, magenta, yellow, and optionally black hue. When a dye-donor element containing three or more primary colour dyes is used, a multicolour image can be obtained by sequentially performing the dye transfer process steps for each colour.
A primary coloured dye layer e.g. a magenta or cyan or yellow dye layer may comprise only one primary coloured dye (a magenta, cyan, or yellow dye respectively) or may comprise a mixture of two or more primary coloured dyes of the same hue (two magenta, two cyan, or two yellow dyes respectively).
All imaging dyes are unstable to light to some extent. Dyes are indeed known to degrade through a number of pathways, which often involve dye triplet states, radicals and/or singlet oxygen. Any improvement in the light stability of dyes is highly desirable therefore.
In photographic silver halide systems various light-stabilizers for dyes are known. For instance UV-absorbers are used frequently in a top layer to shield dyes from the harmful influence of ultraviolet radiation.
Unfortunately, such technique cannot be used in thermal transfer systems, because the dyes, after having been thermally transferred to a receiver sheet, are located in the very top layer of said receiver sheet. If in that case UV-absorbers would be present as well in said top layer, they would intimately mix with the transferred dyes and thus give rise to a so-called catalytic fading effect, which would degrade the dyes even faster.
For use in thermal transfer other types of light-stabilizers such as singlet oxygen quenchers and metal chelates have been described e.g. in EP 312,812. Although these compounds are known to constitute a class of effective stabilizers to light, their performance is not equally adequate for all types of dyes. But above all, the use of such light-stabilizers is undesirable because most of them comprise heavy metal ions, which nowadays raises increasing resistance for ecological reasons.
Furthermore, certain known light-stabilizers cause an undesirable stain in the receiver sheet.